Pickup trucks are motor vehicles with a rear open top cargo area often referred to as a bed. Pickup trucks are popular largely because the bed allows them to be utilized in so many different ways, including carrying a variety of types of cargo and towing various types of trailers. Traditionally the majority of body structures on pickup trucks have been formed from steel alloys. Through years of experience, pickup truck designers have learned how to design steel truck body parts that withstand the variety of demanding pickup truck applications. The current regulatory and economic environment have increased the importance of making pickup trucks more fuel efficient as well as functional and durable. One way to reduce the fuel consumption of a vehicle, especially when unloaded, is to reduce vehicle structure weight.
Aluminum alloys typically have a higher strength to weight ratio than steel alloys. Consequently, replacing steel with aluminum for various vehicle parts offers the potential for weight reduction. However, the elastic modulus of aluminum is generally lower than the elastic modulus of steel. As well, fabrication techniques and methods of joining parts that work well for steel parts may not work well for the same aluminum part. Due to these and other differences, simple material substitution does not necessarily produce an acceptable design. Various methods of joining vehicle parts together to form a vehicle structure are known, each having advantages and disadvantages. Some methods are not suitable for joining a part made of one material to a part made of a different material.
One method suitable for joining parts, including parts made of distinct materials, is a self-piercing rivet process. A self-piercing rivet process is depicted in FIGS. 1 and 2. A stack of parts, including a front part 10 and a rear part 12, is clamped between a die 16 and a blankholder 18. The die includes a protrusion 20. A rivet 22 is forced into the stack by a punch 24. In FIG. 1, the rivet 22 has just begun to deform the materials in the stack. In FIG. 2, the rivet has been forced into the stack far enough that the head of the rivet is approximately flush with the top surface of the front part 10, plastically deforming the front part 10 and the rear part 12. The material that has been pushed into the die during the process is called a button. FIG. 3 shows a cross section through the button. Notice that the rivet 24 has pierced the front part 10 and extends into rear part 12 while splaying open. Mechanical interlock is generated by this rivet splay, where one measure of interlock can be described as the difference in diameters, D2 and D1, divided by 2. D1 is the diameter of the rivet at the interface between the front part 10 and the rear part 12. D2 is the maximum diameter of the splayed rivet tail.
Two failure modes may be encountered in the self-piercing rivet process. In one failure mode, the diameter relationship necessary to interlock the parts is not established. In some circumstances, the non-interlock failure mode may be addressed by using a longer rivet. In a second failure mode, the rear part ruptures at the base of the button. This may expose a surface that is sensitive to corrosion. Also, adhesive that may be applied between the parts may be forced out through the rupture and may collect on the protrusion 20 of the die 16, impacting future joints unless cleaned. In some circumstances, this second failure mode, called breakthrough, may be addressed by using a shorter rivet. In some circumstances, however, such as when the rear part is thin relative to the front part, a rivet long enough to achieve interlock is too long to avoid breakthrough. Consequently, the self-piercing rivet process has traditionally been limited to joints in which the rear part is relatively thick.